The following invention relates to a method and apparatus for installing lightweight ceramic fiber insulation and for supporting electric resistance heating elements in an electric furnace.
Furnaces utilizing electrical resistance heating elements are well known in the art. Typically such furnaces include an outer steel wall of square or cylindrical construction having an inner insulating wall of fire brick or relatively rigid blocks of ceramic insulation. The heating elements for such furnaces typically include a plurality of electric resistance coils wound in a serpentine or sinusoidal fashion and supported from hangers or rods fixedly attached to the fire brick or outer steel wall. In practice it has been necessary with such furnaces to drill holes through the ceramic insulation or brick for the placement of the hangers. An example of such a furnace is shown in British Patent No. 1,433,744 to James H. Nock. In the Nock construction, hooked brackets are inserted through a plurality of insulating layers of material in the interior of a furnace which are affixed to hard refractory material at the outermost casing. The problem with this type of construction is that the bores for the brackets must be precisely placed in order that the separation between adjacent hangers will fit the dimensions of the serpentine heating elements. All bores must be alined within the same horizontal plane and the distance between any two of the bores must be precise so that the upper loops of the heating elements will come to rest on the hangers.
Other furnaces do not use hard refractory material as insulation, but, instead make use of modules of fiberous ceramic batts. The batts are stacked about the interior wall of the furnace contiguously until they form an interior insulating wall. Suitable adhesive, or in some cases, a weldable metallic stud is used on the back of each of the individual batts to secure them to the wall. Brackets for suspending the serpentine heating elements are typically imbedded within the batts. Two examples of this type of construction are shown in the U.S. Patents to Sauder U.S. Pat. Nos. 4,154,975 issued May 16, 1979 and to Carr 4,088,825 issued May 9, 1978. In the furnaces of the '825 and '975 patents, anchors for the support hooks which carry the heating elements are embedded within modules of ceramic fiber insulation. The problem with this type of construction is that in time the anchors will pull free of the insulation. Also, in both patents the heating elements are suspended from ceramic spools supported by a metal rod or hook. If the spool were to crack, the metal underneath would be exposed to the heating element, and heat would be supplied directly to the metal anchors in the interior of the insulation. This is undesireable because it prematurely weakens the insulation covering the anchor, allowing the anchor to pull free.